chore: Add release notes for protobuf 4.x upgrade

libraries-bom/releaseNotes.md

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+### Upgrade Overview
+The `Libraries-Bom` and its managed libraries have been upgraded to use **Protobuf v4.33.2**. All Protobuf gen code included in the managed libraries has been regenerated using `protoc v33.2`. 
+
+---
+### ✅ Backward Compatibility
+The new protobuf-java v4.33.2 runtime is fully backward-compatible with protoc gen code 3.x. You should not experience any incompatibilities from using older gen code with the new runtime.
+Should you experience compatibility issues, please consult the [Known Incompatibilities & Mitigations](#known-incompatibilities-mitigations). If the issue persists, please file a report via the [support console](https://cloud.google.com/support-hub) or our [GitHub repository](https://github.com/googleapis/sdk-platform-java/tree/main)
+
+---
+### ⚠️ Advisory: Vulnerability Warnings
+After upgrading, you may see a new warning in your logs: `Vulnerable protobuf generated type in use`.
+This warning does not mean the Java SDK is vulnerable. It indicates that your application, or one of its other dependencies, is still using gen code < `3.21.7`.
+For a detailed explanation and mitigation steps, please refer to Java section in the official Protobuf [v4.33.0 release notes](https://github.com/protocolbuffers/protobuf/releases/tag/v33.0).
+
+---
+
+### Known Incompatibilities & Mitigations
+While rare, the following incompatibilities may occur due to the version update:
+
+#### 1. Runtime Version Mismatch
+* **Issue:** The minimum required `protobuf-java` runtime version is now **v4.33.2**. Starting with the 4.x runtime, [validations](https://github.com/protocolbuffers/protobuf/blob/1082da2da37a0887d0cfd70abf4a00e8973cd8bf/java/core/src/main/java/com/google/protobuf/RuntimeVersion.java#L55-L73) ensure the runtime version is higher than the gen code version. Using a lower version of protobuf-java will produce errors below.
+* **Symptoms:**
+    * `java.lang.NoClassDefFoundError: com/google/protobuf/RuntimeVersion$RuntimeDomain` (when using 3.x versions).
+    * `com.google.protobuf.RuntimeVersion$ProtobufRuntimeVersionException` (when using older 4.x versions).
+* **Mitigation:** Upgrade the version of `protobuf-java` to be greater than or equal to **v4.33.2**.
+
+#### 2. Class Hierarchy Changes (`GeneratedMessageV3`)
+* **Issue:** `GeneratedMessageV3` is no longer a parent class of gen code.
+* **Symptoms:** Runtime errors will occur if attempting to cast Proto 4.x gen code to `GeneratedMessageV3`. For example, `DeleteInstanceRequest` now extends [GeneratedMessage](https://github.com/googleapis/google-cloud-java/blob/d7a49aa9502012df1209b55ec741b1d4ac639361/java-compute/proto-google-cloud-compute-v1/src/main/java/com/google/cloud/compute/v1/DeleteInstanceRequest.java#L33) instead of [GeneratedMessageV3](https://github.com/googleapis/google-cloud-java/blob/1b56d017b4f93b5037f7261c488e008ac59897d8/java-compute/proto-google-cloud-compute-v1/src/main/java/com/google/cloud/compute/v1/DeleteInstanceRequest.java#L31). You will run into `java.lang.ClassCastException` when trying to run `(GeneratedMessageV3)DeleteInstanceRequest`.
+* **Mitigation:** Migrate usages of `GeneratedMessageV3` to its parent interface, `Message`.
+
+#### 3. Descriptor Syntax APIs
+* **Issue:** Certain internal methods in [Descriptors](https://github.com/protocolbuffers/protobuf/commit/1aeacd4f4eb4e0aa05d6336e2988a565e475e9a0) are no longer available.
+* **Symptoms:** `java.lang.NoSuchMethodError` when calling `Syntax`, `getSyntax()`, `hasOptionalKeyword()` or `isSynthetic()`.
+* **Mitigation:** There are no direct alternative methods; it is suggested to stop using them.
+
+#### 4. Legacy Generated Code (v2.x)
+* **Issue:** The 4.33.x runtime is incompatible with v2.x gen code.
+* **Mitigation:** Migrate gen code to at least **v3.0.0**. Please note that 3.x support will [end in March 2027](https://protobuf.dev/support/version-support/#java); it is strongly recommended to upgrade gen code to 4.x.